NMR measurements are based on providing a net magnetization to a large group of atomic nuclei using a static magnetic field (main field) and deflecting the net magnetization from the direction of the static magnetic field using a radio frequency pulse (RF pulse) of a radio frequency magnetic field (RF field), the operating frequency (Larmor frequency) being defined by the nuclei concerned and the magnitude of the main field. The relaxation of the deflected precessing net magnetization can be detected by measuring the NMR signal, i.e. the EMF induced in the receiving RF coils caused by the precessing net magnetization, which is gradually relaxing back to parallel with the main field (known also as Free Induction Decay, FID)). The relaxation speed is determined by the homogeneity of the main field and the properties of the matter under measurement.
Water content of various material samples can be measured accurately and rapidly using NMR spectroscopy or relaxometry. Wide usage of NMR based moisture content measurement devices has been hindered mainly by the high cost of measurement devices. In particular, in many applications, e.g. biomass water content measurements, the desired sample volume is of the order of several deciliters or larger, which sets practical limits for device dimensions and other specifications.
Prior art [e.g. Remote Automatic On-Line Sensor; Final Report. Quantum magnetics Inc] suggests that a minimum operating frequency of 5 to 6 MHz should be used in NMR-based water content measurements in order to maintain a reasonable recovery time, i.e. receive circuit deadtime following the transmitted RF pulse. This necessitates the use of a relatively high main magnetic fields (>125 mT). High field also increases the measurable signal amplitude. Other prior art [The British Journal of Radiology, 71 (1998), 704-707] also teaches that the electromotive force induced in the receiving coils by the NMR signal is essentially proportional to the square of the main magnetic field strength. However, the use of field strengths of this magnitude increases device costs, power consumption and magnet mass, for example. In addition, the temperature variation and main field inhomogeneity effects are significant in high fields, further complicating device design. Also safety aspects, as concerns e.g. a stray magnetic field outside the device, become more relevant in high magnetic fields. The above aspects are emphasized if the desired sample volume is large. Barak, P. J. et al, “The Use of a Permanent Magnet for Water Content Measurements of Wood Chips”, IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 12, NO. 1, March 2002, present that the water content of wood chips can be measured at a 0.47 T main field, yielding for the operating frequency 20 MHz. The permanent magnet used weighed 68 kg although the possible sample volume (“good field” volume) was only less than 10 ml. Other examples of prior art include water content analysis in dissolution testing, for example as presented by Butler James et al, “Using low-field MRI to improve tablet dissolution testing”, Tablets & Capsules, January 2010 at 0.5 T main field and small sample volume.
There have also been attempts to use lower frequencies and field strengths. For example, Kantzas A. et al, “LOW FIELD NMR APPLICATIONS IN OIL SANDS MINING AND EXTRACTION”, International Symposium of the Society of Core Analysts, Toronto, Canada, 21-25 Aug. 2005, have used a 1 MHz Corespec 1000™ (main field 24 mT produced by permanent magnet) relaxometer apparatus to determine the oil and water content in 20 ml ore and froth samples.
It is relatively easy to produce the required field strength with a good homogeneity over small sample volumes with permanent magnets, as in the prior art referred to above. Large sample volumes are, however, required in many applications, because of sampling standards, as well as the generally large particle dimensions e.g. of biomass samples. The device designs referred to above are, however, not suitable for measuring large sample volumes since the device designs would turn out to be expensive and difficult to manufacture if scaled up to the required size.